Rear Diff Mount - Failed!

[cabrioman]

Re: Rear Diff Mount - Failed!

Interesting. I nearly went with the same type of mount that failed on you. In the end I fitted a standard Ford mount which was filled with Sikaflex to stop the movement you get in the standard mount.

I was going to see how that performed before spending money on a bling mount but thinking about the work involved in fitting a new mount after the event I may go the route you have gone and look for a weldless billet mount.

Definitely think removing the tank is your best option and it will give you the chance to make sure the tank straps are ok, one member, can't remember who, had their tank fall out when the tank straps failed. Many people upgraded the straps after that, myself included.

John

[greyV8pete]

Re: Rear Diff Mount - Failed!

............... Definitely think removing the tank is your best option and it will give you the chance to make sure the tank straps are ok, one member, can't remember who, had their tank fall out when the tank straps failed. Many people upgraded the straps after that, myself included. John

Hi John. Yes there are two options as I see it. 1 Drop the front diff front mounts and move it forward, or 2 Remove the tank as you say. Just trying to avoid that as apart from five pipes to disconnect it is completely full of petrol as I had just topped it up the day before. Although I have 2 jerry cans it will still be a tedious extra job. Peter.

[CosworthCabrioLee]

Re: Rear Diff Mount - Failed!

Hi Peter, done this a few times on cabrios, don't worry I know how you are feeling
Give me a buzz if you want And can talk you through it ?
07720398794
Cheers
Lee

[greyV8pete]

Re: Rear Diff Mount - Failed!

Hi Peter, done this a few times on cabrios, don't worry I know how you are feeling
Give me a buzz if you want And can talk you through it ?
07720398794
Cheers
Lee

Having had a good chat with Lee I have taken his advice and decided to drop the diff assembly to fit my shiny new diff carrier. Many thanks Lee for all the help.

So this afternoon was spent removing the exhausts. The disadvantage of a V8 is after one is off you have to start the struggle all over again under the other side!

While lying under the car I have been pondering the size of the four bolts that attach the rear diff carrier to the car. On mine they are only 8mm. Looking at the Sierra / Granada workshop manual it says 15/18lb/ft torque for these so I guess 8mm is original. Strange as the diff carriers have holes that will accept 10mm bolts. What size bolts does everyone else have?

I then started thinking what loads are imposed on this mount and have come to the following conclusions:

On a Sierra / Granada the diff is mounted on the bent beam subframe which has a three point location on the car. The front two are the big donut things at each front end. The third mounting is the rear diff carrier, which is effectively the rear subframe carrier!

On the production car therefore the top of the rear carrier rubber is under compression with the weight of the car resting on it. Under acceleration the diff will want to drop as a reaction against the drive shaft torque, so theoretically the bottom of the rear carrier rubber will be in compression. But as downwards force is applied to the rear mount the body is also tending to drop with it.

Now consider the situation with a kit car mounting Sierra / Granada diff like the Marlin Cabrio. With our cars there is no subframe beam so the weight of the car is all taken on the rear springs / chassis and rear wishbone / chassis mountings. This means that the diff is effectively “hanging” on the two outrigger bushes and the rear diff mount and the diff mount is being loaded downwards. With the same reaction under acceleration this force is then added to the torque forces due to acceleration. It may also explain why the rubber in the standard Ford mount doesn’t last long, unless the lower void is rubber filled. Does this all make sense?

Either way I have decided to open up the chassis holes and will fit 10mm bolts. I have also made up two straps to fit in the chassis recess in the boot to reinforce this area. This has also been prompted by the observation that after my bracket failure one of the bolts (albeit probably the only remaining one holding prior to final failure) has pulled the chassis area around the hole down a bit. Peter.

[greyV8pete]

Re: Rear Diff Mount - Failed!

Phtos of the shiny new diff mount the new 10mm bolts and the extra reinforcing plates mentioned above. Peter.

[greyV8pete]

Re: Rear Diff Mount - Failed!

Thought for the day! It would be really nice to swap the diff for a 3.36 ratio LSD Granada diff while I have the existing one removed. I know a couple of club members have done this as the V8s can easily pull that ratio and it drops the revs for long distance cruising. Speedo already reads 8% fast, according to sat nav, so it would correct that too!

Problem is finding one of known quality? (and at a sensible price! ).

Interesting link below to Seventh Heaven website which lists diff ratios amongst other useful info.

http://www.super7thheaven.co.uk/Sierra_Rear_Differential

Peter.

[CABRIOBOB]

Re: Rear Diff Mount - Failed!

Pete,
Is your old diff 7" or 7 1/2" as I think I have 2 diffs 3.62 - 7" but not on drugs (lsd) 1 with push in shafts and 1 with bolt on flanges, but I need to check as stored away under other items. I have just noticed your looking for 3.36 ratio, the 3.38 bring daft money well over £300 plus on a std diff without lsd.

Bob

[greyV8pete]

Re: Rear Diff Mount - Failed!

Pete,
Is your old diff 7" or 7 1/2" as I think I have 2 diffs 3.62 - 7" but not on drugs (lsd) 1 with push in shafts and 1 with bolt on flanges, but I need to check as stored away under other items. I have just noticed your looking for 3.36 ratio, the 3.38 bring daft money well over £300 plus on a std diff without lsd.

Bob

Many thanks but mine is already the 3.62 with the LOBRO drive shaft flanges and a Quaife LSD. I need to get down to 3.36 to get some longer legs for the V8 when touring. I am wondering how much Quaife would charge to fit a 3.36 C/W & Pinion to my unit, as I really like the quaife LS principle. Peter.

[greyV8pete]

Re: Rear Diff Mount - Failed!

Further Update! – I have just found out from John at KitFit that as my diff casing has the two long bolts it is 7" not 71/2" as I had previously thought. It does however have the LOBRO driveshaft flanges which according to Lee can be swapped over into non flanged diffs. The good news is that John has a non LSD one which he is sorting out for me. What a star! On reflection I am satisfied to have a more suitable ratio (it will be 3.38:1) and no LSD rather than the 3.62 and the Quaife as I am not really convinced the LSD is needed anyway.

Attached is photo of the “tool” that I cobbled together to access the drive shaft TORX bolts from under the wheel arch. Makes it easier to get to the handbrake in between loosening the bolts two at a time.

LOBRO tool.jpg

[greyV8pete]

Re: Rear Diff Mount - Failed!

Update following an eventful day. Garage removable “extension” (survival tent) fitted and worked extremely well keeping the arctic wind out. The jacking gadget worked like a dream and with a bit of cussing and struggling the diff is finally out.


First checks show that the billet machined unit protrudes several mm further back than the failed unit and so will need some work to make room for it to fit. It never was going to be a straightforward job after all! The new mount came supplied with flat dome head cap screws so I will order some countersink ones tomorrow to gain a couple of mm clearance there.

First off job is moving the fuel tank back. Its straps are so flimsy that it is wobbling around and even hitting on the chassis – another annoying rattle solved! I can support one end at a time on a bit of 4x2 wood whilst fitting new uprated straps. I can see that there is room to move it back about 5mm so the bolt holes in new straps can be chosen to achieve this. After that I can re-assess the clearance gained.

Another option to increase clearance is to remove the backplate from the diff and mill a few mm off the end of the rear mounting face. This would be a last resort as it would then be non-standard. Alternatively, as Lee has suggested to me, I could move the whole diff forward a couple of mm.


Another bonus today is finding that the boot floor on my car has been cut away to reveal all the filler and breather pipes. Is this normal for a Cabrio? Once the car is back on its wheels I can easily remove the fuel sender unit and bend the arm to calibrate the fuel gauge. At present it reads ¾ full with a full tank. I have never risked going below an indicated ¼ full, just in case it is a non-linear scale. The staining on the top of the tank makes me think that the sender unit may not be sealing 100% which may explain the occasional smell of fuel, so it needs to come out anyway.
Peter.
Diff Jack Down.jpgDiff Jack Up.jpgBoot Floor Panels.jpg

[CosworthCabrioLee]

Re: Rear Diff Mount - Failed!

Hi Peter looking good, it may be me but the rear mount looks like a upgraded mount anyway or have you just changed it ? as for the floor my car is the same so you can get easy access to it. Keep up the good work

[greyV8pete]

Re: Rear Diff Mount - Failed!

Hi Peter looking good, it may be me but the rear mount looks like a upgraded mount anyway or have you just changed it ? as for the floor my car is the same so you can get easy access to it. Keep up the good work

Thanks Lee.

The rear mount was allegedly upgraded but unfortunately as the polybush is located much further down from the body mounting plate it has twice the leverage on the mounting plate and those 4 x 8mm bolts. Hence the fatigue cracking around the weld. If I had a machine shop available I would approach the rear support from an entirely different direction. Mike's "koala" type mount is getting there, but still has potential for putting all the stress on the rear cover attachment boss. See my other post in the "General" section of the forum.

I spent the day peeling the notorious powder coating off the diff outriggers. It's annoying when there are stubborn bits in between that just won't move. Breakthrough came when I found that Nitromors paint stripper will soften and lift it easily! So tomorrow is painting day while I eagerly await the arrival of the 3.38 diff.

Also today I bought some 30 x2 mm M/Steel strip and some rubber to make up some new stronger fuel tank straps. If I go on at this rate I will be rebuilding the whole rear end of the car! Peter.

[CosworthCabrioLee]

Re: Rear Diff Mount - Failed!

Hi Peter do you have a close up picture of the old failed carrier it looks to me a non std part, I know it is after market but it still looks wrong.
Cheers lee

[greyV8pete]

Re: Rear Diff Mount - Failed!

Still waiting for my 3.38 diff to arrive so today I finished painting the diff front mounts . Fitted the new countersink screws to the new billet aluminium diff mount and elongated the four top bolt slots 1.5mm to avoid “backwards shift” when I open the chassis hole up from 8mm to 10mm. Tried the mount in position and it now exactly touches the fuel tank. There is about 8mm clearance between the rear of the tank and the chassis crossmember so I will spend tomorrow making up new stronger tank brackets to shift it backwards, provide sufficient clearance for the diff mount, and stop it moving about. I may still have to elongate the front diff mount holes a millimetre or so to suit, but this work should now be minimal. It seems that if the tank moves back by 8mm the filler neck will need rotating a bit to accommodate it and this will also conveniently put the top of the filler cap level with the bodywork as it has always been a bit tilted since I had the car.


As I am still concerned about the non-standard loading on the rear mounting I will also make up the reinforcing strap to locate on the rear pair of bolts. Peter.

[greyV8pete]

Re: Rear Diff Mount - Failed!

Some forward progress at last today. Using a newly made up fuel tank support (plus some ratchet clamps as a safety net) I managed to lower the nearly empty fuel tank about 30mm without disconnecting any fuel hoses. This allowed removal of the top straps for modification. By slotting the rear holes in these and bending the front of the straps I have managed to move the tank back quite a bit. The target dimension of 15mm from the centre of the fixing holes to the inner face of the new bottom straps was needed to have the tank supports vertically in line with the rear chassis rail, this being the maximum possible rearward movement of the tank. With some 3mm skinned foam rubber inside the straps this has produced approximately 3mm clearance at the rear.

The result is that the diff mount has now approximately 2 to 3mm clearance with the new billet machined diff support. I think this is enough as the diff will be well constrained by the new rear polyflex mounting and there is no fore and aft movement of the diff likely, as with the Sierra, as there is no sub-frame floating about on its front rubber donuts. It will also mean that I only have to open the boot / chassis holes out concentrically from 8 to 10mm clearance for the planned larger bolts.


Tomorrow I shall endeavour to make up the reinforcing strap to go under the billet machined rear support whilst waiting for the new diff to arrive. After which as Haynes would say “Reassembly is simply a reversal of the removal process”. Peter.